Most, if not all, instructors use these settings simply because they only use the aircraft in the training area, they never actually go anywhere with it!
Some aircraft have the power setting table reproduced on the sun visor, which is really handy, if not photocopy it and stick it with your nav board, or clipboard.
I always used rule of thumb power settings in the training area, but on navs, I would use the power setting table. Pick the power you want to use (typically 65% or 75%). The setting that is best depends on the hire rate of the aircraft, and the wind at the time.
If you are hiring it "wet" (that is the rate includes the fuel) then effectively the club or company you are hiring from is paying your fuel bill, while you only pay for time - in this case use 75% cruise power to cut your costs.
If you are hiring it "dry", then you will find that 75% power is cheaper in anything greater than about 15 knots headwind.
Once you have decided your power setting, chose the lowest RPM on the chart, with the appropriate MAP. (This may be something like 24"/2200RPM.) This will result in the least noise, and greatest efficiency. If the engine is vibrating too much for comfort, use the next highest RPM setting.